This invention relates to surveying of large structures, and is particularly related to a surveying technique that takes into account optical disturbances, such as those caused by atmospheric turbulence.
Atmospheric turbulence affects optical light paths and introduces a significant amount of error when viewing objects at a long distance in a survey. Standard surveying practices are severly hampered by atmospheric turbulence since the only conventional techniques available to deal with it are mostly qualitative and empirical, and offer solutions that are cumbersome, economically onerous, and impractical.
The methods utilized for modern surveying have attempted to keep the inevitable measurement errors within acceptable limits. The concepts of "accuracy" and "precision" must be understood if typical measurement errors are to be classified. Generally, "accuracy" is defined as "the degree of absolute nearness to the truth", while "precision" means "the degree of refinement in the performance of an operation." There are three types of error which act independently to disturb the accuracy and precision of a survey measurement: Namely, atmospheric conditions, instrument error, and human error.
Typical instrument errors are caused, for example, in tape measures and rods whose lengths are grossly dependent on temperature, and in transits in which the relative location or orientation of the lenses, scales, and levels thereof are subject to temperature variations, manufacturing tolerances, and the physical history of the particular transit. Human errors can be expected, for example, due to the change in focal length of a surveyor's eyes during a survey, or due to fatigue as the work progresses.
In order to carry out a survey in which instrumentation and human errors are brought to within standard tolerances, a combination of geometric measurements, counter measurements, and remeasurements are carried out until the required accuracies are obtained.
Unfortunately, no previous surveying technique has adequately dealt with the problem of inaccuracies resulting from atmospheric disturbances.
One example of an application in which the complexities of surveying techniques have serious impact is in airport runway testing and maintenance. Not only must the runway be set out accurately when it is first installed, but there is a need for recurrent surveys arising from uneven settling of the runways and from support bed deterioration.
The gravity of runway deterioration is determined by the impact of runway uneveness on aircraft. While very short wavelength grade variations (i.e., bumps) constitute the most easily observable deviations from a flat runway, aircraft suspension systems effectively smooth out such variations, and these small grade variations are generally not critical. However, medium wavelength, gradual changes in grade are not absorbed by suspension systems, but rather tend to cause changes in the attitude of the aircraft. This results in changes in the windload forces on the aircraft wing, and if these changes are not anticipated and corrected by the pilot, they can result in serious or disastrous consequences. To make matters worse, these gradual grade variations have always been the most difficult to detect and remove. Furthermore, with the advent of large, tail-heavy planes, the requirement for runway flatness is further increased in that grade wavelengths in excess of 2,000 feet have become critical.
Prior to this invention, it has been very difficult to detect accurately such grade variations over the entire area of a typical runway of, for example, 10,000 foot length and 150 foot width. Typically, an attempt to survey such a runway would require using 20 foot centers, and attaining a first-cut accuracy of within 1/10 foot of vertical error per 1,000 feet measured. The execution of this type survey is painstaking and expensive. Utilizing four-man teams, as much as 500 man-days are required, and the runway must be shut down for the entire period.
However, for all this trouble, the resultant data is often of barely marginal quality. Atmospheric turbulence produces intolerable optical errors at viewing distances in excess of 400 feet on a still winter night, and at distances in excess of 50 feet on a hot summer day. Consequently, the number of transit sightings possible from a single reference transit location is severly limited, and large numbers of transit locations are necessary. The coordinates of each additional transit location can only be determined by dependent calculation, which generates error propagation and thereby renders it virtually impossible to obtain a true accuracy of "less than 1/10 foot error per 1,000 feet" as is required.
Still further, the closing of a runway at a busy airport has an intolerable impact on the operating capacity of the airport. Consequently, airport management is often reluctant to permit such a survey until the surface conditions deteriorate to the point where the hazards presented to aircraft are worse than the traffic congestion problems resulting from a runway shutdown. As a result of this, airport authorities and agencies have long sought an improved surveying technique which will permit accurate measurement of grade variations in the runway, but which will not severly affect runway takeoff and landing operations.
Moreover, a surveying technique has also been sought which will provide an accurate means of spot-checking flatness conditions without interrupting runway operations. However, previous attempts to develop such technique have not been sufficiently successful. Such previously proposed techniques for example include equipping surface vehicles with altimeters, vibration detectors and recorders, and then driving such vehicles down the runways at different speeds. This technique has not provided the desired survey data with the necessary accuracy.
Furthermore, techniques for surveying large structures have been sought without success in allied fields, such as attempting to determine the deflections sustained by large bridges and buildings under severe windload conditions.